This invention pertains to high performance mixed matrix membranes (MMMs). More particularly, the invention pertains to a new type of high performance MMMs incorporating at least two kinds of molecular sieves and methods for making and using the same.
Gas separation processes using membranes have undergone a major evolution since the introduction of the first membrane-based industrial hydrogen separation process about two decades ago. The design of new materials and efficient methods will further advance the membrane gas separation processes within the next decade.
The gas transport properties of many glassy and rubbery polymers have been measured, driven by the search for materials with high permeability and high selectivity for potential use as gas separation membranes. Unfortunately, a key limitation in the development of new membranes for gas separation applications is a well-known trade-off between permeability and selectivity of polymers. By comparing the data of hundreds of different polymers, Robeson demonstrated that selectivity and permeability seemed to be inseparably linked to one another, in a relation where selectivity increases as permeability decreases and vice versa.
Despite concentrated efforts to tailor polymer structure to improve separation properties, current polymeric membrane materials have seemingly reached a limit in the tradeoff between productivity and selectivity. For example, many polyimide and polyetherimide glassy polymers such as Ultem 1000 have much higher intrinsic CO2/CH4 selectivities (αCO2/CH4) (˜30 at 50° C. and 690 kPa pure gas tests) than that of cellulose acetate (˜22), which are more attractive for practical gas separation applications. However, these polyimide and polyetherimide glassy polymers do not have permeabilities that are attractive for commercialization compared to current commercial cellulose acetate membrane products. There are some inorganic membranes, such as zeolite and carbon molecular sieve membranes that offer much higher permeability and selectivity than the polymeric membranes, but are expensive and difficult for large-scale manufacture. Therefore, it remains highly desirable to provide an alternative cost-effective membrane that combines high permeability and selectivity.
Based on the need for a more efficient membrane than the polymer or the inorganic membranes, a new type of membranes, mixed matrix membranes (MMMs), has been developed. MMMs are hybrid membranes containing inorganic fillers such as molecular sieves embedded in a polymer matrix.
MMMs have the potential to achieve higher selectivity with equal or greater permeability compared to existing polymer membranes, while maintaining their advantages. Much of the research conducted to date on MMMs has focused on the combination of a dispersed solid molecular sieving phase such as molecular sieves with an easily processable continuous polymer matrix. For example, see U.S. Pat. No. 6,626,980; US 2005/0268782; US 2007/0022877 and U.S. Pat. No. 7,166,146.
Many organic polymers, including cellulose acetate, polyvinyl acetate, polyetherimide (commercially Ultem®), polysulfone (commercially Udel®), polydimethylsiloxane, polyethersulfone, and several polyimides (including commercial Matrimid®), have been used as the continuous phase in MMMs. Typical inorganic sieving phases in MMMs include various molecular sieves, carbon molecular sieves, and silica. The sieving phase in a solid/polymer mixed matrix scenario can have a selectivity that is significantly higher than the pure polymer. Addition of a small volume fraction of sieves to the polymer matrix, therefore, can increase the overall selectivity significantly.
In recent years, significant research effort has been focused on materials compatibility and adhesion at the inorganic molecular sieve/polymer interface in order to achieve separation property enhancements over traditional polymers with MMMs. For example, Kulkarni et al. and Marand et al. reported the use of organosilicon coupling agent functionalized molecular sieves to improve the adhesion at the sieve particle/polymer interface of the MMMs. See U.S. Pat. Nos. 6,508,860 and 7,109,140. This method, however, has a number of drawbacks including prohibitively expensive organosilicon coupling agents and very complicated time consuming molecular sieve purification and organosilicon coupling agent recovery procedures after functionalization. Therefore, the cost of making such MMMs having organosilicon coupling agent functionalized molecular sieves in a commercially viable scale can be very expensive. Most recently, Kulkarni et al. also reported the formation of MMMs with minimal macrovoids and defects by using electrostatically stabilized suspensions. See US 2006/0117949. U.S. Pat. No. 7,138,006 to Miller et al., entitled “Mixed matrix membranes with low silica-to-alumina ratio molecular sieves and methods for making and using the membranes”, reports the incorporation of low silica-to-alumina (Si/Al) ratio molecular sieves into a polymer membrane with a Si/Al molar ratio of the molecular sieves preferably less than 1.0. Miller et al. claim that when the low Si/Al ratio molecular sieves are properly interspersed with a continuous polymer matrix, the MMM ideally will exhibit improved gas separation performance even without functionalizing the surface of the molecular sieves using organosilicon coupling agent.
Despite all the research efforts, only a few MMMs comprising a continuous polymer matrix and a molecular sieve dispersed therein reported in the literature showed simultaneous enhanced selectivity and permeability for gas separations compared to a corresponding polymer membranes made from the continuous polymer without the molecular sieve. Most of the MMMs exhibited either selectivity improvement with decreased permeability or enhanced permeability without selectivity improvement compared to a corresponding polymer membrane made from the continuous polymer. In high-performance membranes, both high permeability and high selectivity are desirable because higher permeability decreases the size of the membrane area required to treat a given amount of feed, thereby decreasing the capital cost of membrane units, and because higher selectivity results in a higher purity product. While in some cases the polymer “upper-bound” curve has been surpassed using MMMs, it is still highly desirable to provide a new type of MMM that can yield a combination of higher permeability and selectivity over conventional polymer membranes for a variety of applications particularly for gas separations. Huge markets would exist for high volume gas separation membranes if more robust and higher selectivity and permeability MMMs were economically available.